This invention relates to complementary metal oxide semiconductor (CMOS) devices and, more particularly, to CMOS device architectures and processes for manufacturing low voltage, high voltage, or both low voltage and high voltage CMOS devices with a reduced number of processing steps.
Typically, CMOS manufacturing processes require more processing steps than manufacturing processes for standard n-channel metal oxide semiconductor (NMOS) devices. The advantage of reduced power consumption for the CMOS devices compared to NMOS devices is offset by increased manufacturing complexity, i.e., an increased number of manufacturing process steps. The complexity of conventional CMOS manufacturing processes is further increased when both low voltage and high voltage CMOS devices are within the same circuit.
Referring to FIG. 1, a typical low-voltage CMOS device 10 consists of an n-channel (NMOS) portion 12 and a p-channel (PMOS) portion 14 formed on a p-doped substrate 18. (The term substrate, as used herein, refers to one or more semiconductor layers or structures that include active or operable portions of semiconductor devices.) The process steps required to fabricate this device are well-known to those having ordinary skill in the art. For example, one of the first steps in a CMOS fabrication process is to implant and drive in n-well 16 into p-substrate 18. A subsequent step is to mask and implant p-channel stops 20 into substrate 18. Thereafter, thick field oxide 22 is grown over p-channel stops 20, and thin gate oxide 24, 26 is grown over the surface of substrate 18. Polysilicon layer 32, 34 is added atop the thin gate oxide 24, 26, respectively, to form gate structures of the NMOS 12 and PMOS 14 devices, respectively. Finally, in separate mask and implant steps, source/drain (S/D) regions 28 and 30 of the NMOS device 12 and PMOS device 14, respectively, are formed.
The foregoing list of CMOS fabrication steps is not intended to be comprehensive. It is well understood by those having ordinary skill in the art that additional steps are required to fabricate device 10, such as adjusting the threshold voltage (V.sub.t) of the device through doping implantation and forming electrical contacts to substrate 18, n-well 16, S/D regions 28, 30 and gates 32, 34. Moreover, there are many equivalent and suitable known processes for forming such devices. The foregoing steps merely serve to illustrate that typical low-voltage CMOS fabrication requires two separate mask and implant steps to form p-channel stops and S/D regions for p-channel (i.e. PMOS) devices.
CMOS fabrication is further complicated by the combination of high-voltage and low-voltage CMOS devices in a single substrate. Specifically, adding a high-voltage CMOS device to low-voltage CMOS device 10 typically requires yet another set of mask and implant steps. Accordingly, a circuit bearing low and high voltage CMOS devices typically requires three separate sets of mask and implant steps to form p-channel stops, low-voltage PMOS S/D regions and high-voltage PMOS S/D regions.
As a general rule, each mask step increases the complexity of the fabrication process and reduces yield due to the increased potential for processing defects. Increased complexity has a particularly detrimental effect on process yield in high density circuit arrays. Accordingly, it is desirable to eliminate process steps in general, and mask steps in particular, from the steps necessary to fabricate both low and high voltage CMOS devices.